Determination of absorbance for estimating a sample on the basis of the absorbance by detecting the light transmitted through the sample is unsuitable for measuring the absorbance of a thin film having a thickness of not more than the wavelength of infrared, for example, by using infrared. As a method effective for infrared spectroscopy, a high-sensitivity reflectivity measurement has recently been known. The high-sensitivity reflectivity measurement is a method of projecting light onto a sample at an incident angle of not less than 60 degrees and less than 90 degrees, and judging the absorbance of light on the surface of the sample by detecting the reflected light.
In a recent infrared spectroscopy, an FT (Fourier-Transform) system is adopted, and the high-sensitivity reflectivity measurement has attracted more and more attention. The adoption of the FT system to infrared spectroscopy has made the infrared spectroscopy applicable to a microscopic measurement, so that various kinds of infrared microscopic measurement equipments are now commercially available. Various methods which are adopted in general infrared spectroscopes have also been applied to a microscopic measurement.
However, it is very difficult to apply the high-sensitivity reflectivity measurement to general infrared microscopic measurements.
A Casegrainian mirror is usually used as the object mirror of a general infrared microscope. In a high-sensitivity reflectivity measurement using the Casegrainian mirror, since it is necessary that the incident angle of light relative to a sample is 60 to 90 degrees, the Casegrainian mirror must have a large angular aperture.
It is possible to produce a Casegrainian mirror having a large angular aperture in principle, but the production is technically very difficult. In addition, since a Casegrainian mirror having a large angular aperture diffuses light in a wide range, the utilization efficiency of light is greatly lowered in comparison with a general Casegrainian mirror. The application of a high-sensitivity reflectivity measurement equipment to a microscopic measurement has not been generalized because an objective mirror type measurement equipment is required in a microscopic measurement, which increases the limitation at the time of installation in comparison with a general measurement equipment. A reflective mirror, especially, a conventional Casegrainian mirror is mostly used as the object mirror of an infrared microscope, but the Casegrainian mirror has a complicated structure and costs high.
The same problems are produced in the case of using a unit equipment adopting the high=sensitivity reflectivity measurement.